The direct synthesis of methylchlorosilanes from silicon and methyl chloride at 250.degree. to 300.degree. C. by means of copper catalysts produces, in addition to the methylchlorosilanes of the general formula Me.sub.x SiC.sub.4, x having values of from 0 to 4 and Me represents a methyl group, also ethylchlorosilanes, various hy silanes, in particular Me.sub.y HSiCl.sub.3-y, y having values from 0 to 2, and ethyldichlorosilane (EtHSiCl.sub.2) in small amounts. Furthermore, various straight-chain and branched alkanes and alkenes having up to 9 carbon atoms are also formed as impurities. The direct synthesis is described, inter alia, in W. Noll, 2nd Edition 1968, Verlag Chemie, Weinheim, Chapter 2.2.
The methylchlorosilanes are separated by distillation and freed from impurities. However, some alkenes cannot be completely removed in this manner due to their boiling point or because of the formation of azeotropic mixtures. Co-entrained alkenes, in particular branched alkenes, undergo a reversible addition reaction with hydrogen chloride upon hydrolysis of the methylchlorosilane to intermediates or end products. Due to their high boiling point, the chloroalkanes formed therefrom remain in the product and slowly release hydrogen chloride when the product is heated during its use. This hydrogen chloride causes, for example, as condensation catalyst undesirable reactions.
The most desirable product of the direct synthesis is Me.sub.2 SiCl.sub.2, which can be reacted by hydrolysis and polycondensation to form silicone polymers having a variety of functional groups and structures.
An essential feature of the quality of most silicone polymers is a minimum amount of trifunctional impurities in the polymer structure. The most frequent trifunctional impurities of the Me.sub.2 SiCl.sub.2 used are MeSiCl.sub.3 and EtHSiCl.sub.2.
Due to these impurities, the distillation of Me.sub.2 SiCl.sub.2, which in most cases is carried out continuously, requires apparatuses and equipment having very high separation efficiencies, since the boiling points of the components differ only slightly.
The mode of operation and the quality control are monitored by gas chromatography. However, the EtHSiCl.sub.2 content is very difficult to detect, since the GC signal is superimposed by that of trans-3-methylpentene even with optimum analysis of gas chromatographic methods. This alkene by-product of direct synthesis is very difficult to separate off despite its slightly different boiling point relative to Me.sub.2 SiCl.sub.2 and even accumulates in Me.sub.2 SiCl.sub.2 fractions. Table I shows the most important boiling points and concentrations of a high-quality Me2SiCl.sub.2 fraction. Hereinafter the concentrations are always by weight.
TABLE I ______________________________________ Silane Boiling point Concentration ______________________________________ Me.sub.2 SiCl.sub.2 70.degree. C. &gt;99.9% MeSiCl.sub.3 66.degree. C. &lt;1 ppm EtHSiCl.sub.2 71.degree. C. &lt;1 ppm trans-3-methylpentene 68.degree. C. &lt;1 ppm ______________________________________
Apart from the troublesome behavior described above of co-entrained alkenes, trans-3-methylpentene elevates the analytical values of the EtHSiCl.sub.2 content. This requires high reflux ratios during distillation, which lead to a high energy consumption and reduced capacity of the equipment.
Therefore, it is an object of the present invention to provide a process for removing alkenes during methylchlorosilane distillation.